Maintenance - yes, if you did a 20-25 mile run every 3-4 weeks that would be even better IMHO. If you keep "marathon fit" then you'll be in great shape to start an ultra training plan when ready.
I did FIRST when training for my first marathon, it's a tough programme - all runs are tempo/interval or long at near marathon pace. Plus you're supposed to do 2 cross training sessions on top, so it's run less, run faster but not necessarily train less, run faster.
Endurance is slow to fade, so keeping your aerobic base is easier than maintaining speed which fades quickly, (at least mine has) keep a few short fast sessions and do a long(er) run every few weeks should be OK at least in the short/mid term...
12.5 hours is still a bit low for an all-nighter in the winter, I'd definitely want a backup battery!
As an aside - I've recently been using on old Alpkit Gamma as a handtorch by wrapping the headband around my wrist and placing the lamp unit over my knuckles. I really like being able to direct the beam without having to look that way, I also find it you get less "focussed" in your own little bubble of light.
I had one of the old Nao's and wasn't impressed with the battery life - it let me down badly in the middle of the night (twice). I may have just had a duff one, and it was well below freezing both times which may have contributed - but it ended up on eBay last year.
I've now got a Silva Trailrunner II which I love and has a nice diffused beam, it also takes normal AAA batteries. I like the idea of rechargeable headtorches for shorter events, but if you want one that's going to last through 12+ hours and/or multiple nights then I'm always going to go for replaceable batteries AND a backup, (e-Lite or similar)